This invention relates to a process for the purification of 1,3-dihalobenzenes, particularly 1,3-dichlorobenzene, and their separation from a mixture of corresponding 1,4- and 1,3-dihalobenzenes. The purification of 1,3-dihalobenzene is accomplished by selectively removing the 1,4-isomer as a complex with polyethylene glycol. The process according to the invention produces 1,3-dihalobenzenes in high purity.
1,3-dichlorobenzene is a commercially important intermediate compound which serves as a building block in the synthesis of many products that are important to both the pharmaceutical and agricultural industries. The direct synthetic routes to produce 1,3-dichlorobenzene are limited and are economically unfeasible. Normally, 1,3-dichlorobenzene is co-produced along with other dichlorobenzenes by the chlorination of benzene. Similarly, dichlorobenzenes can be isomerized to a mixture containing predominantly 1,3-dichlorobenzene.
The recovery of 1,3-dichlorobenzene from mixtures of other dichlorobenzenes, particularly 1,4-dichlorobenzene, is one of the most difficult separations in aromatic chemistry. Normally, the separation and/or purification of these organic chemicals is carried out by distillation or crystallization. In the case of 1,3- and 1,4-dichlorobenzene, separation via the distillation route is practically impossible due to their narrow difference in the boiling points (less than 0.2.degree. C. apart from one another). Similarly, the purification of 1,3-dichlorobenzene from a mixture of 1,4- and 1,3-dichlorobenzenes by crystallization is also impossible due to the formation of an eutectic mixture where both compounds freeze at the same temperature. The eutectic point for 1,4- and 1,3-dichlorobenzene mixture is approximately -29.4.degree. C. and consists of approximately 88% 1,3-dichlorobenzene and 12% 1,4-dichlorobenzene. Up to or closer to the eutectic point, 1,4-dichlorobenzene can be removed from the mixture in high purity by crystallization. However, in the proximity of the eutectic point, removal of 1,4-dichlorobenzene by crystallization becomes difficult and therefore the purity of 1,3-dichlorobenzene can not be improved further. At eutectic point and lower temperatures, both 1,4- and 1,3-dichlorobenzene freezes together. If the composition changes such that the mixture contains more than 88% 1,3-dichlorobenzene, the crystallization at or above -29.4.degree. C. will yield pure 1,3-dichlorobenzene. But to achieve the concentration of 1,3-dichlorobenzene required for its purification through crystallization is very difficult.
Because of the difficulty in the separation and purification of 1,3-dichlorobenzene, several complicated and expensive options have been suggested and commercially practiced to produce high purity 1,3-dichlorobenzene. For example, the reaction of metachloronitrobenzene with SOCl.sub.2 (Beilstein V-243, V1-(129), metabenzenesulfonic acid with SOCl.sub.2 (Beilstein XI-68, XI1-(21), and metabromonitrobenzene with PCl.sub.5 (Beilstein V-248, V1-(131) have been described.
U.S. Pat. No. 3,170,961 describes a process of extracting 1,3-dichlorobenzene from a mixture of 1,4- and 1,3-dichlorobenzenes by a bromination route. In this process, during the bromination step, the 1,3-isomer is selectively reacted to form 1-bromo-2,4-dichlorobenzene which is separated from the 1,4-dichlorobenzene by distillation. The bromodichlorobenzene is debrominated in a second step in the presence of a bromine acceptor, e.g. benzene, and aluminum chloride as the catalyst. This process is complicated and expensive and produces undesirable by-products such as hydrogenbromide gas and bromobenzene.
Other purification processes, such as the one described in U.S. Pat. No. 2,958,708, include the physical separation of the 1,3-isomer with various agents, such as a molecular sieve. While this process provides the desired purity of 1,3-dichlorobenzene, the regeneration of the molecular sieve is very difficult. This process is also not cost effective.
Japanese patent JP 89,313,446 (application JP 01,313,446) describes a process to purify 1,3-dichlorobenzene by forming an inclusion complex between this isomer and a host compound, such as 9,9'-bianthracene, and distillation of the complex from 1,4-dichlorobenzene at a temperature lower than the decomposition point of the complex. This process has several shortcomings, such as requiring the handling of toxic compounds and lack of cost effectiveness.
U.S. Pat. No. 4,996,380 describes another process for separating 1,3-dichlorobenzene from a mixture of dichlorobenzenes using the selective absorption characteristics of certain zeolites. While high purity 1,3-dichlorobenzene can be produced by the use of this technique, this process becomes expensive since the zeolites can not be used indefinitely. Also, an additional solvent (which needs to be removed subsequently through distillation) is required to extract the absorbed 1,3-dichlorobenzene.
Another method to separate 1,3-dichlorobenzene from a mixture of 1,4- and 1,3-dichlorobenzenes is described in German patent 2,855,940. It uses combination of distillation and crystallization. In this process the crude dichlorobenzene mixture is distilled to first increase the concentration of the 1,3-dichlorobenzene to approximately 90%. The distillate is then subjected to a crystallization to extract the pure 1,3-dichlorobenzene. After the separation of the 1,3-dichlorobenzene, the mother liquor is recycled to the distillation stream to further increase its purity to 90%. Even though this method produces high purity 1,3-dichlorobenzene, it requires a distillation column with a large number of stages. Therefore, this process becomes very difficult to practice and economically unattractive.
Belgium patent BE 897,296 describes a process for the concentration of 1,4-dichlorobenzene using a thin film (100-u) of very high molecular weight (5.times.10.sup.6) glycols to increases its concentration to 70%. However, high purity 1,3-dichlorobenzene is not produced. Moreover, such high molecular weight thin film glycols are not commercially available.
German patent 23 32 889 and European patent application 0 451 720 describe methods to separate 1,3-dichlorobenzene from a mixture containing 1,3- and 1,4-dichlorobenzenes. Both processes use an extractive rectification technique using different extractants. For instance, the German patent uses hexamethyl phosphoric acid triamide and the European patent uses alkylene carbonates. Various other compounds, such as dimethyl sulfoxide, n-ethyl pyrolidone and dibutyl sulfoxide, have been described as possible extractants for this process. However, some of these substances are toxic, corrosive, unstable, and their boiling points unfavorable for high temperature distillation.
In view of the deficiencies of the aforementioned prior art processes, it is highly desirable to provide a new process for the purification and commercial production of high purity 1,3-dihalobenzenes from a mixture containing its corresponding 1,4-disubstituted isomer.